I'VE worked several years in aircraft radio shops and
am a Commercial Pilot with both Airplane and Helicopter
ratings.

The primary target is the reflection of the ground
transmitted radar signal at approximately 1215 mHz by the
body of the aircraft. This reflection is almost always
weaker than the secondary target. The secondary target is
the signal sent back to the ground by the transponder in
the aircraft at 1090 mHz in response to a ground query at
1030 mHz.

This system was developed at the close of WWII and is
known in the military as IFF. (Identification Friend or
Foe) Civil aircraft are controlled primarily by such
means. A pattern of pulses selected by the pilot are sent
back to give the controllers a unique identifying number.
In most cases the aircraft's altitude is sent back as
well. The suicide pilots had turned off this system
leaving only the primary return.

This was done to decrease the information available to
the controllers and reduce their warning time. I've done
it myself many times when joy riding to keep the
controllers from knowing my altitude. Civilian FAA
controllers use the term target constantly to refer to
any radar signal that represents an aircraft. It has
nothing to do with any fighter intercept activity.